Electrophotographic image forming apparatuses such as laser printers, electrostatic copying machines, plain paper facsimile machines and printer-copier-facsimile multifunctional machines have been widely prevalent with constant improvement, for example, for higher speed image formation, higher quality image formation, full-color image formation and size reduction. The improvement will be continuously made from now on.
For example, the laser printers are required to have a further reduced size and a maintenance-free feature for further prevalence thereof in the future, and research and development are constantly conducted for this purpose. To follow this trend, a developing roller to be incorporated in such a laser printer is also required to have a further reduced size.
In the developing section of the laser printer, toner is brought into contact with an outer peripheral surface of a roller body of the developing roller at a predetermined pressure by a quantity regulating blade to be thereby electrically charged and adhere to the outer peripheral surface. The toner adhering to the outer peripheral surface of the roller body is transported to a surface of a photoreceptor body by rotation of the developing roller to be thereby brought into contact with an electrostatic latent image formed on the surface of the photoreceptor body. Thus, the electrostatic latent image is developed into a toner image.
In many laser printers, the developing roller is provided together with the photoreceptor body and a toner container in the form of a cartridge, which is removably mounted in a laser printer housing. When toner in the cartridge is used up, the cartridge including the developing roller and the photoreceptor body is replaced with a new one. Thus, the laser printer is substantially free from maintenance.
The developing roller is typically produced by forming an electrically conductive rubber composition into a cylindrical body and crosslinking the rubber composition to form a roller body, and inserting a shaft such as of a metal into a center through-hole of the roller body to electrically connect and mechanically fix the shaft to the roller body.
The electrically conductive rubber composition is prepared, for example, by blending a rubber component including at least a copolymer rubber (ion-conductive rubber) containing ethylene oxide as a comonomer and having an ion conductivity and an acrylonitrile butadiene rubber (NBR), and additives such as a crosslinking agent and a crosslinking accelerating agent for crosslinking the rubber component.
In order to meet the recent requirement for further size reduction of the laser printer and to develop a compact full-color laser printer, the size of the cartridge should be further reduced.
For this purpose, the developing roller should satisfy the following requirements:
Reducing the diameter of the developing roller;
Reducing the hardness of the roller body to increase the flexibility of the roller body in order to reduce a load to the toner;
Reducing the friction coefficient of the outer peripheral surface of the roller body as much as possible in order to reduce the load to the toner; and
Reducing the hardness of the roller body to maintain the compression set of the roller body at a lower level (suppressing so-called “permanent compressive deformation”).
Among these requirements, the reduction of the toner load is important for so-called higher durability laser printers which are adapted to form a great number of images by using toner contained in a single cartridge.
That is, a very small part of the toner contained in the cartridge is used in a single image forming cycle, and the remaining major part of the toner is repeatedly circulated in the cartridge. Since the developing roller is provided in the cartridge and repeatedly brought into contact with the toner, the magnitude of the load (damage) to be applied to the toner by the developing roller, if any, is a critical factor that determines how long the image formation quality can be properly maintained when the same toner is repeatedly used for the image formation.
If the load is too great, the toner is liable to be deteriorated to have a broader particle size distribution due to disintegration or agglomeration of toner particles or suffer from fluctuation in charging characteristics, thereby causing a fogging phenomenon in a formed image. The fogging phenomenon is such that the deteriorated toner is spread over the background of the formed image to reduce the image quality.
As disclosed in Patent Literature 1, for example, it is effective to use a styrene butadiene rubber (SBR) capable of forming a more flexible crosslinking product than the conventional NBR in combination with the ion conductive rubber for the rubber component in order to increase the flexibility of the roller body for reduction of the toner load.
It is also effective to further blend a chloroprene rubber (CR) as the rubber component in order to control the roller resistance of the developing roller and the compression set of the roller body.
For the reduction of the friction coefficient of the outer peripheral surface of the roller body, it is effective to form an oxide film in the outer peripheral surface of the crosslinked roller body through oxidation of the crosslinking product of the electrically conductive rubber composition forming the outer peripheral surface by irradiating the outer peripheral surface of the roller body with ultraviolet radiation in an oxidative atmosphere.
The oxide film is formed through the oxidation of the crosslinking product of the electrically conductive rubber composition forming the outer peripheral surface of the roller body. Therefore, the oxide film is more uniform in thickness and surface geometry than a coating layer of the prior art formed, for example, by applying a given coating agent on the outer peripheral surface of the roller body without a problem such that dust and other foreign matter are caught in the coating layer during the formation of the coating layer.
Since the oxide film can be easily formed through the oxidation of the crosslinking product of the electrically conductive rubber composition forming the outer peripheral surface of the roller body by the irradiation with the ultraviolet radiation without additionally preparing the coating agent, it is possible to suppress the reduction in the productivity of the developing roller and the increase in production costs.